Step-down switching DC-DC converters are used to supply power-supply voltages to various electronic devices. Such a DC-DC converter often includes a rectifier that reduces rectification loss by performing synchronous rectification. A switch element, such as a metal oxide semiconductor (MOS) transistor, may be used as the rectifier.
FIG. 22 illustrates a conventional step-down DC-DC converter 4 that performs synchronous rectification. The step-down DC-DC converter 4 includes a main transistor T11, a synchronous transistor T12, a coil L11, a smoothing capacitor C11, and a control circuit 5 for controlling activation and inactivation of the transistors T11 and T12 in a substantially complementary manner.
The step-down DC-DC converter 4 alternately activates and inactivates the main transistor T11 and the synchronous transistor T12 to maintain an output voltage Voa at a target voltage. For example, the main transistor T11 is activated in response to a high (H) level control signal DHa to supply energy from an input terminal to an output terminal. Further, the main transistor T11 is inactivated to release energy accumulated in the coil L11. The synchronous transistor T12 is activated in response to an H level control signal DLa generated in synchronization with the timing at which the energy accumulated in the coil L11 is released to a load. The duty ratio of a pulse signal for driving the main transistor T11 is determined by the ratio of an input voltage Vi and the output voltage Voa. However, the DC-DC converter 4 executes feedback control in accordance with the output voltage Voa or feedback control in accordance with the output voltage Voa and an output current Io in a current control mode. This maintains the output voltage Voa at the target voltage. Examples of such feedback control include pulse width modulation (PWM) control and pulse frequency modulation (PFM) control.
When the load state of this synchronous rectification step-down DC-DC converter 4 suddenly changes from a heavy load to a light load or no load, the step-down DC-DC converter 4 would supply the load with power exceeding the required amount. In this case, the output voltage Voa increases and overshoots the target voltage.
Japanese Laid-Open Patent Publication No. 2008-125226 describes a technique for inactivating the main transistor T11 and activating the synchronous transistor T12 when the output voltage Voa increases to a certain voltage. This decreases the amount of current flowing through the output terminal Po and prevents the output voltage Voa from increasing. With the technique described in the publication, the synchronous transistor T12 is inactivated when the overshooting output voltage Voa decreases to the target voltage.
With the technique described in the publication, an amount ΔVoa of overshoot of the output voltage Voa produced when the load state suddenly changes may be expressed by the following equation, in which Voa is the voltage value of the output voltage Voa, ΔIo is the amount of change of the output current Io, L11 is the inductance value of the choke coil L11, and C11 is the capacitance value of the capacitor C11.
                                          Δ            ⁢                                                  ⁢            Voa                    =                                    Δ              ⁢                                                          ⁢              Io              ×              t                                      2              ×              C              ⁢                                                          ⁢              11                                      ⁢                                  ⁢        whereas        ⁢                                  ⁢                  t          =                                    L              ⁢                                                          ⁢              11              ×              Δ              ⁢                                                          ⁢              Io                        Voa                                              (        1        )            
With the technique described in the above publication, the synchronous transistor T12 is activated after the overshooting occurs. As a result, the amount of current flowing through the output terminal Po decreases gradually. This decreases the overshooting amount ΔVoa of the output voltage Voa. However, a gradient along which the coil current IL changes is determined by V0/L during the activated period of the transistor T12. As a result, the overshooting amount ΔVoa is decreased by a relatively small degree.